An electrophotographic image-forming device well known in the art, such as a laser printer, forms images on a recording medium by exposing a photosensitive drum to a laser beam emitted from an optical scanner (scanning device) to carry a developer image on the photosensitive drum, and by transferring the developer image from the photosensitive drum to the recording medium such as a sheet or a transparency.
A color laser printer includes a plurality of photosensitive drums corresponding to each color employed in the printer. After the scanning device scans each photosensitive drum, forming an electrostatic latent image on the photosensitive drum, developer is supplied to the latent image to form a developer image on the photosensitive drum. Subsequently, a color image is formed on a recording medium by superimposing each of the developer images on the recording medium (see Japanese patent application publication No. 2006-178372).
As is clear from the above description, if the electrostatic latent images are not formed precisely on the photosensitive drums when the drums are exposed by the scanning device, color registration problems will appear in the resulting image formed on the recording medium.
Further, the scanning device has an optical scanning system configured of a polygon mirror or other deflector, fθ lenses, and reflecting mirrors for guiding a laser beam emitted from a light source configured of a semiconductor lasers to the photosensitive drum. Hence, the laser beam scanned over the photosensitive drum is greatly affected by the dimensional tolerance, mounted positions, and adjustment tolerance of the deflector and the optical scanning system.
Normally, the effects of variations in the dimensions of the deflector and the optical scanning system and variations in the mounting positions of the same can be absorbed by measuring properties of the laser beam scanned over the photosensitive drum (the scanning linearity of scan line and trajectory of scan line) with a special measuring device and by adjusting the timing for emitting the laser beam based on the results of the measurement.
However, since the laser emitting timings of the laser beams must be adjusted based on the results of measuring properties related to the laser beam (hereinafter referred to as laser beam properties) after the scanning device is mounted in the printer, the scanning device cannot easily be replaced or repaired.
In other words, it is difficult to replace of repair the scanning device without taking the device to a service center equipped with the measuring device required for measuring laser beam properties (hereinafter referred to as a special service center). Hence, if the laser printer is already in the possession of the end-user, for example, the end-user must transport the printer to a special service center to replace or repair the scanning device, making it difficult to provide the end-user with prompt maintenance and service.